The present invention is related to capacitors comprising a solid conductive polymeric electrolyte and an optional liquid electrolyte. More specifically, the present invention is related to a capacitor comprising a conductive separator and a method of forming the hybrid capacitor with improved conductive polymer coverage within the interstitial portions of a wound structure.
Capacitors have historically been defined within two general types with one type utilizing a liquid electrolyte and the other type utilizing a solid electrolyte. Liquid electrolyte capacitors, generally, comprise a layered structure typically as a winding with an anode conductor, a cathode conductor and an interleaved separator immersed in a liquid electrolyte all sealed within a container. Solid electrolyte capacitors, generally, include a conductive monolith or foil with a dielectric layer thereon and a solid cathode, such as conductive polymer or manganese dioxide, on the dielectric. Both general types of capacitor have experienced wide spread use in commerce and each has advantages, and disadvantages, not common to the other. For example, liquid electrolytic capacitors have a high capacitance but a poor Equivalent Series Resistance (ESR) due to poor conductivity of the electrolyte, typically not above about 0.015 S/cm, whereas conductive polymers have a high conductivity, up to 600 S/cm, and therefore capacitors utilizing conductive polymeric cathodes have a much lower ESR.
Conductive polymeric cathodes have seen wide spread use in commerce due, at least in part, to their low equivalent series resistance (ESR) and non-destructive failure mode. This has lead to a desire to form a hybrid capacitor wherein the conductive polymers commonly employed for solid electrolytic capacitors are utilized within the windings of a liquid electrolyte structure with the goal of achieving the high voltage common with liquid electrolyte capacitors while maintaining the lower ESR common with solid conductive polymeric electrolytes. U.S. Pat. Nos. 8,462,484 and 8,767,377 teach exemplary hybrid capacitors.
The formation of a hybrid capacitor has typically involved the formation of the interleaved wound structure; comprising anode, cathode and separator; followed by impregnation with the conductive polymer. The impregnation has been done by either in-situ polymerization of monomers, or by diffusion of pre-formed polymer slurry into the interstitial areas of the wound interleaved structure.
In-situ polymerization of a monomer in the presence of an oxidizer was used to manufacture a first generation of hybrid capacitors. In-situ polymerization is a complex method with many problems including contamination of the final product by monomer and oxidizer and the work environment conditions are complex leading to poor process reliability. These issues were mitigated by the use of water based dispersions, or slurries, of pre-formed conductive polymer to impregnate the interstitial spaces of the capacitor winding.
Impregnation of a winding with preformed conductive polymer involves dipping the working element into a solution comprising conductive polymer or adding the solution onto the working element wherein the conductive polymer migrates, or diffuses, into the interstitial spaces. Manufacturing stages are complicated due to limitations associated with the rate and efficiency of diffusion through the working element. Filtration of polymeric particles and counterions by the separator limits effective diffusion thereby limiting the length of the working element. As a result, only small capacitor sizes have been successfully achieved. Large capacitors have proven difficult to make. In fact, the maximum case size widely available commercially is about 10 mm in diameter and about 12.5 mm length with a maximum capacitance of about 22 ρF (at rated voltage 63V) and the lowest ESR achieved is about 16 mil.
Due to manufacturing limitations, hybrid capacitors have been primarily radial capacitors as the manufacturing process is not suitable for small axial capacitors. With axial capacitors the bottom tab, or lead, will necessarily be dipped in the polymer precursor or polymer slurry thereby resulting in a polymer coated tab which causes problems with subsequent processing. Furthermore, it is virtually impossible to apply voltage across the capacitor, such as to form polymer in-situ or to heal damaged sites, since the bottom tab will be in the solution.
In spite of the ongoing efforts, those of skill in the art still do not have a suitable method for forming a hybrid capacitor, and a hybrid capacitor formed thereby, suitable for large case sizes or suitable for use in manufacturing axial capacitors. The present invention provides a method for making a hybrid capacitor which is not size dependent and which exhibits improved quality and reproducibility.